The present invention relates to ink printing, particularly to printing of cans, and particularly relates to an inking system for the printing apparatus. In the printing industry, productivity advancement is important. Most productivity advances have come through increased operating speed. Inking arrangements have used the fundamental roller to roller method virtually since modern inking systems were devised. Physics and practicality limit the size and numbers of rollers in such a system.
In a typical roller to roller inking system, ink is supplied to a first roller, e.g. a ductor roller, and from there the ink is transferred to other rollers in succession at nips between adjacent rollers. The ink application roller or form roller or form rollers preceding the printing plate cylinder in the series should be covered with rubber or a rubber-like material. The printing plate is on a cylinder that forms a nip with the form roller or forming rollers and the ink receiving elements on the surface of the printing plate cylinder are formed of metal or a metal-like material. The printing plate cylinder, in turn, transfer ink to a printing xe2x80x9cblanketxe2x80x9d which then prints on an article, e.g. a can.
Inking systems typically use rubber covered or rubber-like material covered rollers alternating with steel or other metal-like hard covered rollers in a series of rollers leading to the printing plate cylinder. When the rubber-like rollers contact the metal-like rollers at nips, the surfaces of the rubber-like rollers deflect and displace at the contact nips for reducing the probability of roller surface damage and controlling ink thickness on the rollers. As the rubber or rubber-like material rotates out of the nip, it deflects again to restore its original shape. Rubber or rubber-like material covered rollers are adjustable during system operation to be set at appropriate contact pressures against the adjacent metal rollers. Since each rubber roller is typically between adjacent metal rollers, this may require a time-consuming compound adjustment of several rollers. A rubber roller or rubber-like surface on a roller which is in pressure contact with the adjacent metal rollers, experiences friction from the texture of the adjacent metal rollers. Over time, a rubber, or rubber-like roller surface tends to wear so that the roller or its surface must be replaced frequently, and the wearing requires frequent roller position adjustment as the roller diameter changes through wear. As existing inking systems are complex with multiple parts, there are related high manufacturing and maintenance and adjustment costs.
The ink from an ink supply tends to adhere to each roller surface after the ink contacts the roller and after a nip has been passed, which causes the ink to be eventually transmitted to the printing plate cylinder.
Although ink tends to adhere to a roller surface after contact, the modulus of elasticity of the ink is eventually exceeded after each nip, due to the speed of roller rotation, the deflection of the roller surface and separation of the roller surfaces after they pass through a contact nip. The contact of adjacent rollers at a nip when the rollers are rotating rapidly rollers separate after the inked surfaces rotate out of the nip. The ink is in tension during the separation and splits when its tension limit is exceeded. The splitting causes the ink to form mist, and to also form airborne particles which are slung and may be dropped to adjacent rollers and may texture a roller surface at which the ink splitting occurs. This may contaminate and/or produce a textured ink surface on roller surfaces and may contaminate adjacent areas of the inking system. In addition, the mist contaminates the air and surrounding equipment. This situation is aggravated at higher operating speeds due to increasing centrifugal force at the roller surfaces, which is likely to sling more of the ink and create more mist. The ink is both slung and split at each roller surface separation following a nip. If the atomized ink is not controlled, it spreads around the inking system and its parts and can become a health hazard. It is not uncommon for an inking system to have nine or even more positions at which an ink split occurs. Further, a typical can printer apparatus may have several separate inking systems operating simultaneously to apply ink from their printing plate cylinders to a printing blanket. This multiplies the ink split problem.
Many printing presses have overhead shrouds or chambers with induced suction with the intent of capturing ink particles. It is not practical to attempt to recycle the recaptured ink because it is a mixture of various pigments and chemistry, and the waste ink must be disposed of ecologically safely, which is a costly process. Reducing the amount of waste ink to be collected is desirable.
Most inking systems use two or more rubber covered form rollers to transfer ink from the preceding distributing rollers to the printing plate cylinder surface. Form rollers typically have relatively small diameters due to space limitations that are inherent in inking system designs. Form rollers must receive compound adjustments so that they can contact both the ink distributing rollers and the printing plate cylinder at the same time. Because of their relative sizes as compared to the printing plate cylinder, the form rollers often make more than one revolution for each revolution of the printing plate cylinder, which leads to the xe2x80x9cghostingxe2x80x9d or xe2x80x9chaloxe2x80x9d, printing effect discussed below.
The printing plate at the end of an inking system is normally wrapped on a printing plate cylinder or roller. The printing plate is in many cases a relief plate, with raised surface areas that accept ink from the form rollers and with recessed areas to which ink is not to be transferred. The raised areas of the printing plate eventually indent the form rollers with the printing plate image. Those indented areas on the form roller make it difficult for the ink distribution roller to apply ink uniformly and evenly to the printing plate via the form rollers. The uneven distribution of ink on the printed substrate causes a xe2x80x9cghostingxe2x80x9d or xe2x80x9chaloxe2x80x9d effect. Ghosting occurs when two similar images are offset from each other. One solution is to provide multiple form rollers of different diameters to help reduce the ghosting. This adds to material, manufacture and complexity of operation costs and increase maintenance.
Each time a different matter or different color is printed, it is necessary to change the printing plates and/or the ink colors used in the inking system. During changing of ink colors, the ink distributing rollers must be cleaned to avoid contamination of the new color by the previous color. Semi-automatic cleaning systems for the rollers do not assure complete cleaning, so that some hand cleaning is required. It is time consuming and can be dangerous to the operator. Belt type inking systems reduce the number of rollers that must be cleaned.
The foregoing describes problems experienced with conventional roller to roller inking systems.
But, inking systems using belts entrained over rollers for ink transfer and distribution are known in the art. One example is disclosed in U.S. Pat. No. 2,036,451, which shows a belt for ink transfer entrained around guide rollers within the loop of the belt and also provided with an ink distribution roller partially wrapped around by the external ink carrying surface of the belt and located along the belt path between two nips formed with the printing plate cylinder. Although the benefit of ink distribution for eradicating a pattern left during previous contact with the plates is disclosed in this prior art patent, operation of the ink distribution roller as disclosed below is not suggested.
Multiple belt type inking systems used in a single printing apparatus are disclosed in U.S. Pat. Nos. 536,077; 773,444; 1,691,795; 3,366,056; 4,593,617. Other belt type ink distribution arrangements are found in U.S. Pat. Nos. 2,622,522 and 4,993,321. In none of these references is the ink distribution roller driven or moved as disclosed below.
Accordingly, one object of the invention is to minimize ink splitting and at least substantially reduce ink slinging off the rollers, and/or mist creation.
Another object of the invention is to improve the ink transfer onto the printing plate cylinder, which is required for high-quality printing.
Another object is to eliminate or substantially reduce xe2x80x9cghostingxe2x80x9d or xe2x80x9chaloxe2x80x9d printing.
The present invention includes an endless loop belt having one ink receiving, ink carrying and ink transferring surface outside the belt loop and an opposite drive surface inside the belt loop. The belt is entrained over a plurality of guide rollers which engage the drive surface of the belt and guide the belt. At least one ink distribution roller engages the ink carrying surface of the belt after ink has been supplied to that surface and before the contact of that surface with the printing plate cylinder for distributing the ink over the ink carrying surface.
The ink carrying belt, the guide and drive rollers for the belt and the printing plate cylinder all travel at the same surface velocity in one direction.
The ink distribution roller is a smooth surface roller to enhance the ink distribution and make it more uniform, and to avoid ghosting and halo printing. In one preferred version, the ink distribution roller travels in the one direction at a surface velocity that is different than, i.e., either faster or slower than, the surface velocity of the ink carrying surface of the belt which passes over the ink distribution roller. In particular, the ink distribution roller is recommended to travel at a surface velocity that is in the range of 2% to 40% faster or slower than the velocity of the ink carrying belt. This tends to distribute the ink uniformly and at the correct thickness over the ink carrying surface of the belt.
Further, the ink carrying roller is vibrated or oscillated axially while revolving around its fixedly located rotation axis. This also assures proper ink distribution and eliminates the halo effect on the belt caused by the continued contact of the printing plate cylinder with the ink carrying surface of the belt.
To control the speed at which the ink carrying belt is driven, at least one of the guide rollers for the ink carrying belt is driven, although more than one of those rollers may be driven. The roller(s) is driven by a power source, preferably mechanically coupled to the main drive of the decorator. Each of the guide rollers is toothed around its periphery complementary to toothing of the drive surface of the belt. The toothing may also be profiled to prevent the belt from shifting laterally or axially, particularly under the influence of the axial vibration of the ink distribution roller.
The benefits of the invention include possible avoidance of use of rubber covered rollers and elimination of the repeated replacement and adjustments required when such material rollers are used; reducing ink splitting by a significant amount; reducing the number of nips or locations where the ink is spread and squeezed and opening of nips which would lead to splitting and slinging of ink; reducing ghosting or halo images; reducing the amount of maintenance and cleaning that are normally required; and reducing manufacturing costs.
With the present invention, when the ink is changed, only the ink distribution roller requires cleaning along with any other rollers which are outside the belt loop. But, the rollers inside the belt loop do not require cleaning, reducing the clean up required between ink color changes.
The clean up of the system of the invention is simple in that the ink transfer belt is removed, the ink distribution roller is cleaned and a new belt is installed. The removed belt can be cleaned apart from the operation of the inking system and a new belt can be immediately installed, minimizing the down time of operation while a belt is off the machine.
Although it is not intended to restrict the applications for the inking system of the invention, it is designed for use in continuous can printing and handling apparatus which applies decoration to the exteriors of cylindrical containers or cans while the containers are mounted on respective mandrels disposed along the periphery of a large, rotating, wheel-like carrier. An example of such an application is found in U.S. Pat. No. 5,111,742, to the assignee hereof. Several separate inking systems, each a system according to the invention, are arrayed around a large diameter printing blanket cylinder, and the blanket is inked by the printing plate cylinder of each of the inking systems. The blanket then transfers the ink to successive containers or cans to be decorated which are presented to the blanket by the individual mandrels, as the blanket and the array of mandrels rotate or move past one another. For application of the inking system of the invention to a can printer, U.S. Pat. No. 5,111,742 is incorporated by reference. Because several inking systems are used, for example, in one embodiment, perhaps as many as eight or nine inking systems, at the printing blanket, the danger of the ink splitting and contamination described above is multiplied and any arrangement to reduce that is desired. Each inking system is in effect an individual machine installed on a can handling system. The inking system is an intricate and delicate device with numerous wearable parts, like bearings, rubber rollers, adjusting mechanisms, and ink contamination is a major source of wear and maintenance problems.